(1) FIELD OF THE INVENTION
THIS INVENTION relates to air cushion vehicles (A.C.V.) and is particularly concerned with means for varying the geometry of inflatable skirt structures forming at least a part of the vehicle cushion sealing means.
(2) DESCRIPTION OF THE PRIOR ART
The cushion sealing means on an A.C.V. may include inflatable skirt structures depending below the main hull of the vehicle, whose primary purpose is to contain the air cushion; inflatable keel structures, extending lengthwise of the vehicle below the underside of the main hull for increasing the roll stability of the vehicle; and inflatable compartmentation structures, extending laterally of the vehicle below the underside of the main hull to divide the air cushion area into fore and aft compartments, for increasing the pitch stability of the vehicle.
In order to effect directional and/or trim control of an A.C.V. it may be necessary to move one or more of the inflatable skirt structures either towards and away from, or laterally, relative to the surface over which the vehicle travels. Thus, for example, fully flexible skirted A.C.V.'s tend to roll outwardly under the action of centrifugal force when turning about a remote point. Amongst the various means that may be used to counteract this tendency and to assist in directional control of the A.C.V. are lift vectoring devices. These devices can be divided into two categories; those which physically move the cushion and hence the centre of pressure; and those which modify the cushion pressure distribution thus changing the centre of pressure position and hence lift moment about the centre of gravity. Both of these types may be used to provide rolling and/or pitching moments and hence side and thrust forces, and should surface contact occur at forward speed, some yawing moment. By effecting inward movement of the skirt hemline along that side of the A.C.V. which is on the inside of a turn, the position of the centre of pressure of the cushion can be shifted with respect to the centre of gravity to produce a lift moment that counteracts the tendency of the vehicle to roll outwardly. Alternatively, or additionally, the hemline of the skirt may be lifted along that side of the A.C.V. which is on the inside of the turn, so as to allow cushion air to escape, thereby modifying the cushion pressure distribution so that the position of the centre of pressure is shifted.
Heretofore, movement of the flexible skirt for such purposes has generally been effected by mechanical means such as cables and pulleys, or levers and rod arms, operated by hydraulic jacks. Examples of such systems are to be found in U.S. Pat. Nos. 3,288,235 and 3,481,423. Such systems introduce considerable complexity at the underside of the vehicle, which is exposed to a very corrosive environment when the vehicle is operating over a sea surface. This has given rise to problems such as failure of the hydraulic jacks, due to sticking or leakage of hydraulic fluid. Another disadvantage with such systems is that unless a longitudinally extending rigid rod is interposed in the system between the hydraulic jacks and the connection to the flexible skirt, the hydraulic jacks will not give equal movement of the hemline of the skirt over that length of the skirt on which they are acting. Also, in order that cushion pressure may be effective over substantially the complete area of the underside of the rigid base structure of the A.C.V., the inflatable skirt structure may project outwardly from the peripheral sides of the A.C.V. rigid body structure. This is advantageous in that the skirt structure is able to deflect in an upward direction, away from a ground or water surface over which the A.C.V. is operating, without being substantially obstructed by the rigid body structure. However, it increases the overall width dimension of the A.C.V. which may give rise to problems when it is required to manoeuvre the A.C.V. into a restricted opening such as a hangar building or the well deck of a carrier ship.
It has been proposed in U.S. Pat. Nos. 3,467,215 and 3,373,839 to provide inflatable skirt structures that project outwardly of the sides of the A.C.V. rigid body structure when inflated and which collapse inwardly towards the rigid body structure when deflated so as to reduce the overall width dimension of the A.C.V. when it is non-operational. However, this does not permit the A.C.V. to be moved under its own power and it has been further proposed in U.K. Pat. No. 1,494,733 to provide an A.C.V., having inflatable skirt structure similar to that disclosed in U.S. Pat. No. 3,467,215, with supporting wheels whereby it may be moved when not cushion supported by the propulsive means of the A.C.V. supplemented, if desired, by diverting pressurised air from the vehicle lift fan to reaction outlets to produce reaction thrust. These wheels may be retractable into recesses in the rigid base structure so as not to obstruct the cushion area in cushion-borne operation. However, the weight of the wheels and any retraction mechanism reduces the payload that the vehicle is able to carry and adds complexity at the underside of the rigid base structure where it is most likely to suffer damage.